Carbon monoxide production from carbon dioxide reduction by elemental sulfur

ABSTRACT

Disclosed is a method of producing carbon monoxide (CO) and sulfur dioxide (SO 2 ), the method comprising obtaining a reaction mixture comprising carbon dioxide gas (CO 2 (g)) and elemental sulfur gas (S(g)), and subjecting the reaction mixture to conditions sufficient to produce a product stream comprising CO(g) and SO 2 (g).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit to U.S. Provisional Patent ApplicationNo. 62/065,427, filed Oct. 17, 2014, titled “CARBON MONOXIDE PRODUCTIONFROM CARBON DIOXIDE REDUCTION BY ELEMENTAL SULFUR”.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention generally concerns the production of carbon monoxide,sulfur dioxide, and carbonyl sulfide from the reduction of carbondioxide by elemental sulfur. The produced carbon monoxide can then beconverted into synthesis gas (syngas) and other valuable chemicals,while the other reaction products can be used to produce additionaleconomically viable chemicals (e.g., both sulfur dioxide and carbonylsulfide can be used to produce fertilizers).

B. Description of Related Art

Carbon dioxide is a relatively stable and non-reactive molecule whencompared with carbon monoxide. Carbon monoxide is more interesting inthis respect, as it can be used to produce several downstream chemicalproducts. For instance, syngas (which includes carbon monoxide andhydrogen gases) is oftentimes used to produce chemicals such asmethanol, tert-butyl methyl ether, ammonia, fertilizers, 2-ethylhexanol, formaldehyde, acetic acid, and 1-4 butane diol.

Syngas can be produced by common methods such as methane steam reformingtechnology as shown in reaction equation (1), partial oxidation ofmethane as shown in reaction (2), or dry reforming of methane as shownin reaction (3):

CH₄+H₂O→CO+3H₂ ΔH_(298K)=206 kJ  (1)

CH₄+O₂→CO+2H₂ ΔH_(298K)=−8 kcal/mol  (2)

CH₄+CO₂→2CO+2H₂ ΔH_(298K)=247 kJ  (3)

While the reactions in equations (1) and (2) do not utilize carbondioxide, equation (3) does. Commercialization attempts of the dryreforming of methane have suffered due to high energy consumption,catalyst deactivation, and applicability of the syngas compositionproduced in this reaction. Equation (4) illustrates the catalystdeactivation event due to carbonization.

CH₄+2CO₂→C+2CO+2H₂O  (4)

Other attempts to convert carbon dioxide into carbon monoxide includethe catalyst reduction of carbon dioxide using hydrogen as shown inequation (5).

CO₂+H₂→CO+H₂O ΔH=10 kcal/mol  (5)

This process, which is also known as a reverse water gas shift reaction,is mildly endothermic and takes place at temperatures at about 450° C.However, commercialization of this process suffers from the hydrogenavailability. In particular, hydrogen is relatively expensive to produceand isolate. Thus, the present costs and sources of hydrogen are notfavorable on a commercial scale to convert CO₂ to CO per equation (5).

While other attempts have been made to produce carbon monoxide fromcarbon dioxide, these attempts have also proven to be inefficient. Forinstance, U.S. Pat. No. 1,793,677, utilizes carbonaceous fuel (e.g.,coal), carbon dioxide, and oxygen to produce a mixture consisting ofcarbon monoxide together with small quantities of carbon dioxide andsulfurous anhydride. The sulfurous anhydride is a by-product due to theminimal amounts of suphur present in the coal. In particular, carbondioxide and oxygen are passed over the carbonaceous fuel at temperaturesgreater than 1000° C. The main source of carbon in this reaction is coalrather than carbon dioxide. Carbon acts as a reducing agent whichreduces carbon dioxide and oxygen to carbon monoxide. Also, both oxygenand carbon dioxide acts as oxidizing agents. Therefore, the primaryreactants are carbon, oxygen and carbon dioxide, and sulfur is asecondary reactant as its concentration in coal is very low. Sulfur isoxidized to sulfurous anhydride by either oxygen or carbon dioxide.Ultimately, the carbonaceous fuel source as well as the reactiontemperatures adds cost and complexities to producing carbon monoxide.

Still further, Asadi et al., in Nature Communications 5, 2014, Vol. 5pp. 4470, describes an electrochemical reduction of carbon dioxide usinga molybdenum disulfide as a catalyst at 54 mV in an ionic liquid toproduce carbon monoxide and hydrogen. The complexity and additionalcomponents needed to drive this reaction also results in an inefficientprocess that is not commercially viable for producing carbon monoxide.

SUMMARY OF THE INVENTION

A solution to the problems associated with the production of carbonmonoxide from carbon dioxide has been discovered. In particular, thesolution resides in the ability to reduce carbon dioxide with elementalsulfur gas to produce carbon dioxide and sulfur dioxide as shown inreaction equation (6):

2CO₂(g)+S(g)→2CO(g)+SO₂(g) ΔH=−8 kJ/mol  (6)

Carbonyl sulfide (COS) can also be produced by this reaction. Thisreaction can be carried out at relatively low temperatures (e.g., attemperatures at which substantial vapor pressure of S exists, e.g.,vapor pressure of S is 5×10⁻⁴ atm at 119° C. and 1 atm at 444.6° C.) andwithout the assistance of water, oxygen, or hydrogen. Therefore, lowerenergy requirements are needed to run the reaction, and the costs andcomplexities associated with introducing water, oxygen, or hydrogen intothe reaction can be avoided altogether. Notably, the methods of thepresent invention can minimize natural gas consumption, can utilizecarbon dioxide produced as a byproduct in the production of manypetrochemicals, and can economically convert carbon dioxide andelemental sulfur into value added chemical products (e.g., CO, SO₂, andCOS).

In one particular aspect of the invention, a method of producing carbonmonoxide (CO) and sulfur dioxide (SO₂) from a reaction mixturecomprising carbon dioxide gas (CO₂(g)) and elemental sulfur gas (S(g))is described. The method can include subjecting the reaction mixture toconditions sufficient to produce a product stream comprising CO(g) andSO₂(g). The carbon monoxide can be obtained from reduction of the carbondioxide by elemental sulfur. The reaction mixture can include a molarratio of CO₂(g):S(g) of 1:1, 2:1, or 4:1, or 1:1 to 6:1. In someinstances, the reaction mixture can consist essentially of or consist ofCO₂(g) and S(g). In a particular aspect, the reaction mixture does notinclude hydrogen gas, oxygen gas, methane gas, and water. In someinstances, the product stream comprises CO(g) and SO₂(g) and one or moreother products such as carbonyl sulfide (COS(g)), carbon disulfide(CS₂(g)), CO₂(g), S(g), or any combination thereof. In some aspects ofthe invention the product stream consists essentially of or consists ofCO(g), SO₂(g), COS(g), CO₂(g), and S(g) or CO(g), SO₂(g), COS(g),CS₂(g), CO₂(g), and S(g). In some instances of the present invention,the product stream does not include CS₂(g). Without wishing to be boundby theory, it is believed that carbon disulfide is not produced from thereaction mixture when the molar ratio of CO₂(g):S(g) ranges from 4:1 to6:1. Process conditions to effect the production of carbon monoxide andsulfur dioxide include a temperature of at least 250° C. or at least445° C., or from 250° C. to 3000° C., 445° C. to 3000° C., preferably900° C. to 2000° C., most preferably 1000° C. to 1600° C., a pressure of1 to 25 bar, and a gas hourly space velocity (GHSV) of 1,000 to 100,000h⁻¹. In some aspects of the invention a catalyst is used to catalyze thereduction of carbon dioxide. The catalyst can be a bulk metal catalyst,a supported catalyst, or both. The catalyst can include a metal, a metaloxide, a lanthanide, a lanthanide oxide, or any combination thereof.Non-limiting examples of the metal or metal oxide includes a Group IIA,IB, IIB, IIIB, IVB, VIB, or VIII metal and/or metal oxide. Non-limitingexamples of lanthanides or lanthanide oxide includes La, Ce, Dy, Tm, Yb,Lu, CeO₂, Dy₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, or La₂O₃, or any combinationthereof. The catalyst support can include a metal sulfide, a metalcarbide, a metal nitride, or a metal phosphate, or any combinationthereof. In some aspects of the invention, the catalyst is MoS₂ or ZnS.In some aspects of the invention, the method can further includeisolating the reaction products from the reaction mixture (such aspassing the reaction mixture through a membrane separation system orseparating the reaction mixture using condensation process such ascryogenic distillation). The carbon monoxide can be isolated andconverted into syngas. The SO₂(g) can be isolated and converted toSO₃(g), and the SO₃(g) can be subsequently converted to sulfuric acid orammonium sulfate. In some embodiments, the isolated COS can be recycledto the reactor at a temperature of 900° or more. At such temperatures,further production of COS from CO and S is inhibited.

In one aspect of the invention, a system for producing carbon monoxide(CO) and sulfur dioxide (SO₂) is described. The system can include aninlet or a first inlet and a second inlet, a reactor, and an outlet influid communication. The inlet can be for a feed comprising in a carbondioxide gas (CO₂(g)) and elemental sulfur gas (S(g)) or a first inletfor a feed comprising CO₂(g) and a second inlet for a feed comprisingS(g). The reactor can include a reaction zone that is configured to bein fluid communication with the inlet, the outlet or both. In someinstances, a continuous flow reactor, for example, a plug-flow reactoror a fluidized reactor, can be used. The outlet can be configured to bein fluid communication with the reaction zone to remove a product streamcomprising CO(g) and SO₂(g). The reaction zone can include CO₂(g) andS(g) CO(g) and SO₂(g), COS(g), or any combination thereof. In someaspects of the invention, the reaction zone can further include acatalyst capable of catalyzing the conversion of CO₂(g) and elementalsulfur gas S(g) into CO(g) and sulfur dioxide SO₂(g). The system canfurther include apparatus capable of separating the individual productsfrom the product mixture. Non-limiting examples of separationapparatuses includes a condenser apparatus capable of condensing theproduced SO₂(g) to SO₂(l) and separating the SO₂(l) from the producedCO(g) and COS(g), a membrane apparatus capable of separating CO(g) fromCOS(g), and a scrubber apparatus capable of separating CO(g) from traceamounts of COS(g) and SO₂(g). The condenser apparatus can be downstreamfrom the reactor, the membrane apparatus can be downstream from thecondenser apparatus, and the scrubber apparatus can be downstream fromthe membrane apparatus. Other non-limiting examples of separationapparatuses include a condenser apparatus capable of condensing theproduced SO₂(g) to SO₂(l) and separating the SO₂(l) from the producedCO(g) and COS(g), and a cryogenic distillation apparatus capable ofcondensing the COS(g) to COS(l), and separating the COS(l) from theCO(g), and a scrubber apparatus capable of separating CO(g) from traceamounts of COS(g) and SO₂(g). The condenser apparatus can be downstreamfrom the reactor, and the cryogenic distillation apparatus is downstreamfrom the condenser apparatus, and the scrubber apparatus is downstreamfrom the cryogenic distillation apparatus. The system can include anoutlet in fluid communication with a COS/CO separation system (e.g., themembrane system) and configured to produce COS from a COS/CO stream. Thesystem can also include a COS inlet in fluid communication with theoutlet of the separation system and the reactor.

In yet another embodiment there is disclosed a reaction mixturecomprising carbon dioxide gas (CO₂(g)) and elemental sulfur gas (S(g)).The reaction mixture can consist essentially of or consist of CO₂(g) andS(g). The molar ratio of CO₂(g) to S(g) can be 1:1 to 6:1, 2:1 to 6:1,3:1 to 6:1, or 4:1 to 6:1. The molar ratio of CO₂(g) to S(g) can be 1:1.The molar ratio of CO₂(g) to S(g) can be 2:1. The molar ratio of CO₂(g)to S(g) can be 4:1. The molar ratio of CO₂(g) to S(g) can be 6:1. Incertain instances, hydrogen gas, oxygen gas, methane gas, or water, orany combination thereof, or all thereof, are not present in the reactionmixture.

Also disclosed is a product stream comprising carbon monoxide gas CO(g)and sulfur dioxide gas SO₂(g). The product stream can further includecarbonyl sulfide gas (COS(g)) or carbon disulfide gas (CS₂(g)) or both.The product stream can further include carbon dioxide gas (CO₂(g)) andelemental sulfur gas (S(g)). The product stream can consist essentiallyof or consist of CO(g), SO₂(g), COS(g), CO₂(g), and S(g). The productstream can consist essentially of or consist of CO(g), SO₂(g), COS(g),CS₂(g), CO₂(g), and S(g). In certain instances, the product stream doesnot include CS₂(g).

In the context of the present invention 65 embodiments are described. Ina first embodiment, a method of producing carbon monoxide (CO) andsulfur dioxide (SO₂) is described. The method can include (a) obtaininga reaction mixture comprising carbon dioxide gas (CO₂(g)) and elementalsulfur gas (S(g)); and subjecting the reaction mixture to conditionssufficient to produce a product stream comprising CO(g) and SO₂(g).Embodiment 2 is the method of embodiment 1, wherein the product streamfurther comprises carbonyl sulfide (COS(g)). Embodiment 3 is the methodof embodiment 2, wherein the product stream further comprises carbondisulfide (CS₂(g)). Embodiment 4 is the method of any one of embodiments1 to 3, wherein the product stream further comprises CO₂(g) and S(g).Embodiment 5 is the method of embodiments 4, wherein the product streamconsists essentially of or consists of CO(g), SO₂(g), COS(g), CO₂(g),and S(g) or CO(g), SO₂(g), COS(g), CS₂(g), CO₂(g), and S(g). Embodiment6 is the method of any one of embodiments 1 to 5, wherein the reactionmixture comprises a CO₂(g):S(g) molar ratio of 1:1 to 6:1. Embodiment 7is the method of embodiment 6, wherein the reaction mixture comprises aCO₂(g):S(g) molar ratio of 1:1. Embodiment 8 is the method of embodiment6, wherein the reaction mixture comprises a CO₂(g):S(g) molar ratio of2:1. Embodiment 9 is the method of embodiment 6, wherein the reactionmixture comprises a CO₂(g):S(g) molar ratio of 4:1. Embodiment 10 is themethod of embodiment 9, wherein the product stream does not includeCS₂(g). Embodiment 11 is the method of embodiment 6, wherein thereaction mixture comprises a CO₂(g):S(g) molar ratio of 6:1. Embodiment12 is the method of embodiment 11, wherein the product stream does notinclude CS₂(g). Embodiment 13 is the method of any one of embodiments 1to 12, wherein the reaction temperature in step (b) is at least 445° C.Embodiment 14 is the method of claim 13, wherein the reactiontemperature in step (b) is 250° C. to 3000° C., preferably 900° C. to2000° C., most preferably 1000° C. to 1600° C. Embodiment 15 is themethod of any one of embodiments 1 to 14, wherein the reaction pressureis 1 to 25 bar. Embodiment 16 is the method of any one of embodiments 1to 15, wherein a gas hourly space velocity (GHSV) of 1,000 to 100,000h⁻¹ is used. Embodiment 17 is the method of any one of embodiments 1 to16, further comprising contacting the reaction mixture in step (b) witha catalyst. Embodiment 18 is the method of embodiment 17, wherein thecatalyst comprises a metal, a metal oxide, a metal sulfide, alanthanide, a lanthanide oxide, or any combination thereof. Embodiment19 is the method of embodiment 18, wherein the metal, metal oxide, ormetal sulfide includes a Group IIA, IB, IIB, IIIB, IVB, VIB, or VIIImetal. Embodiment 20 is the method of embodiment 19, wherein the metalsulfide comprises molybdenum or zinc. Embodiment 21 is the method ofembodiment 21, wherein the lanthanide, or lanthanide oxide includes La,Ce, Dy, Tm, Yb, Lu, CeO₂, Dy₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, or La₂O₃, or anycombination thereof. Embodiment 22 is the method of any one ofembodiments 17 to 21, wherein the catalyst is a bulk metal catalyst.Embodiment 23 is the method of any one of embodiments 17 to 22, whereinthe catalyst is a supported catalyst. Embodiment 24 is the method ofembodiment 23, wherein the support comprises a metal sulfide, a metalcarbide, a metal nitride, or a metal phosphate, and any combinationthereof. Embodiment 25 is the method of any one of embodiments 1 to 24,wherein hydrogen gas, oxygen gas, methane gas, and water are notincluded in the reaction mixture. Embodiment 26 is the method of any oneof embodiments 1 to 25, wherein the produced CO(g) is isolated andconverted into syngas. Embodiment 27 is the method of any one ofembodiments 1 to 26, wherein the produced SO₂(g) is isolated andconverted to SO₃(g), and the SO₃(g) is subsequently converted tosulfuric acid. Embodiment 28 is the method of any one of embodiments 1to 27, wherein the product stream comprises CO(g), SO₂(g) and COS(g),and one or more of the products are isolated by a condensation process.Embodiment 29 is the method of embodiment 28, wherein the isolatedproduct comprises SO₂(g). Embodiment 30 is the method of any one ofembodiments 1 to 29, wherein the product stream comprises CO(g), SO₂(g)and COS(g), and one or more of the products are isolated by a membraneseparation process. Embodiment 31 is the method of embodiment 30,wherein the isolated product comprises COS(g), CO(g), or both.Embodiment 32 is the method of any one of embodiments 1 to 31, whereinthe product stream comprises CO(g), SO₂(g) and COS(g), and one or moreof the products are isolated by a liquid scrubbing process. Embodiment33 is the method of embodiment 32, wherein the isolated productcomprises CO(g). Embodiment 34 is the method of any one of embodiments 1to 27, wherein the product stream comprises CO(g), SO₂(g) and COS(g),and one or more products is isolated by a cryogenic distillationprocess. Embodiment 35 is the method of embodiment 34, wherein theisolated products comprise COS(g), CO(g) or both. Embodiment 36 is themethod of any one of embodiments 1 to 27, wherein the product streamcomprises CO(g), SO₂(g) and COS(g), and one or more components areisolated using one or more processes from embodiments 28-34. Embodiment37 is the method of any one of embodiments 1 to 27, wherein the productstream comprises CO(g), SO₂(g) and COS(g), and the COS (g) is recycledto step (b) at a reaction temperature of 900° C. or more. Embodiment 38is the method of embodiment 37, wherein recycling the COS (g) inhibitsformation of additional COS (g). Embodiment 39 is the method of any oneof embodiments 1 to 38, wherein the reaction in step (b) is anexothermic reaction.

Embodiment 40 describes a system for producing carbon monoxide (CO) andsulfur dioxide (SO₂). The system can include an inlet for a feed thatcan include a carbon dioxide gas (CO₂(g)) and elemental sulfur gas(S(g)) or a first inlet for a feed that can include CO₂(g) and a secondinlet for a feed comprising S(g); and a reactor that includes a reactionzone that is configured to be in fluid communication with the inlet orinlets, wherein the reaction zone comprises CO₂(g) and S(g); and anoutlet configured to be in fluid communication with the reaction zone toremove a product stream comprising CO(g) and SO₂(g). Embodiment 41 isthe system of embodiment 40, wherein the reaction zone further comprisesCO(g) and SO₂(g). Embodiment 42 is the system of embodiment 41, whereinthe reaction zone further comprises COS(g). Embodiment 43 is the systemof any one of embodiments 40 to 42, further comprising a collectiondevice that is capable of collecting the product stream. Embodiment 44is the system of any one of embodiments 40 to 43, wherein the reactionzone can further include a catalyst capable of catalyzing the conversionof CO₂(g) and elemental sulfur gas S(g) into CO(g) and sulfur dioxideSO₂(g). Embodiment 45 is the system of any one of embodiments 40 to 44,wherein the reactor is a plug-flow reactor or a fluidized reactor.Embodiment 46 is the system of any one of embodiments 40 to 45, furthercomprising a condenser apparatus capable of condensing the producedSO₂(g) to SO₂(l) and separating the SO₂(l) from the produced CO(g) andCOS(g), a membrane apparatus capable of separating CO(g) from COS(g),and a scrubber apparatus capable of separating CO(g) from trace amountsof COS(g) and SO₂(g). Embodiment 47 is the system of embodiment 46,configured such that the condenser apparatus is downstream from thereactor, the membrane apparatus is downstream from the condenserapparatus, and the scrubber apparatus is downstream from the membraneapparatus. Embodiment 48 is the system of any one of embodiments 40 to45, further comprising a condenser apparatus capable of condensing theproduced SO₂(g) to SO₂(l) and separating the SO₂(l) from the producedCO(g) and COS(g), and a cryogenic distillation apparatus capable ofcondensing the COS(g) to COS(l), and separating the COS(l) from theCO(g), and a scrubber apparatus capable of separating CO(g) from traceamounts of COS(g) and SO₂(g). Embodiment 49 is the system of embodiment45, configured such that the condenser apparatus is downstream from thereactor, and the cryogenic distillation apparatus is downstream from thecondenser apparatus, and the scrubber apparatus is downstream from thecryogenic distillation apparatus. Embodiment 50 is the system of any oneof embodiments 46 to 49, further comprising: (i) an outlet configured tobe in fluid communication with the membrane to remove COS from themembrane; and (ii) an inlet for the COS configured to be in fluidcommunication with the membrane outlet and the reactor.

Embodiment 51 is a reaction mixture that can include carbon dioxide gas(CO₂(g)) and elemental sulfur gas (S(g)). Embodiment 52 is the reactionmixture of embodiment 51, consisting essentially of or consisting ofCO₂(g) and S(g). Embodiment 53 is the reaction mixture of any one ofembodiments 51 to 52, wherein the molar ratio of CO₂(g) to S(g) is 1:1to 6:1. Embodiment 54 is the reaction mixture of embodiment 51, whereinthe molar ratio of CO₂(g) to S(g) is 1:1. Embodiment 55 is the reactionmixture of embodiment 51, wherein the molar ratio of CO₂(g) to S(g) is2:1. Embodiment 56 is the reaction mixture of embodiment 51, wherein themolar ratio of CO₂(g) to S(g) is 4:1. Embodiment 57 is the reactionmixture of embodiment 51, wherein the molar ratio of CO₂(g) to S(g) is6:1. Embodiment 58 is the reaction mixture of any one of embodiments 51to 57, wherein hydrogen gas, oxygen gas, methane gas, and water are notincluded in the reaction mixture.

Embodiment 59 is a product stream that includes carbon monoxide gasCO(g) and sulfur dioxide gas SO₂(g). Embodiment 60 is the product streamof embodiment 59, further including carbonyl sulfide gas (COS(g)).Embodiment 61 is the product stream of embodiment 60, further includingcarbon disulfide gas (CS₂(g)). Embodiment 62 is the product stream ofany one of embodiments 59 to 61, further comprising carbon dioxide gas(CO₂(g)) and elemental sulfur gas (S(g)). Embodiment 63 is the productstream of embodiment 62, consisting essentially of or consisting ofCO(g), SO₂(g), COS(g), CO₂(g), and S(g). Embodiment 64 is the productstream of embodiment 63, consisting essentially of or consisting of orCO(g), SO₂(g), COS(g), CS₂(g), CO₂(g), and S(g). Embodiment 65 is theproduct stream of any one of embodiments 59 to 64, wherein the productstream does not include CS₂(g).

The term “about” or “approximately” are defined as being close to asunderstood by one of ordinary skill in the art, and in one non-limitingembodiment the terms are defined to be within 10%, preferably within 5%,more preferably within 1%, and most preferably within 0.5%.

The term “bulk metal oxide catalyst” as that term is used in thespecification and/or claims, means that the catalyst includes one ormore metals, or metal oxides/metal sulfides or metal nitrides and doesnot require a carrier or an inert support.

The term “substantially” and its variations are defined as being largelybut not necessarily wholly what is specified as understood by one ofordinary skill in the art, and in one non-limiting embodimentsubstantially refers to ranges within 10%, within 5%, within 1%, orwithin 0.5%.

The terms “inhibiting” or “reducing” or “preventing” or “avoiding” orany variation of these terms, when used in the claims and/or thespecification includes any measurable decrease or complete inhibition toachieve a desired result.

The term “effective,” as that term is used in the specification and/orclaims, means adequate to accomplish a desired, expected, or intendedresult.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims or the specification may mean “one,” but itis also consistent with the meaning of “one or more,” “at least one,”and “one or more than one.”

The words “comprising” (and any form of comprising, such as “comprise”and “comprises”), “having” (and any form of having, such as “have” and“has”), “including” (and any form of including, such as “includes” and“include”) or “containing” (and any form of containing, such as“contains” and “contain”) are inclusive or open-ended and do not excludeadditional, unrecited elements or method steps.

The methods of the present invention can “comprise,” “consistessentially of,” or “consist of” particular ingredients, components,compositions, etc. disclosed throughout the specification. With respectto the transitional phase “consisting essentially of,” in onenon-limiting aspect, a basic and novel characteristic of the methods ofthe present invention are their abilities to produce carbon monoxide andsulfur dioxide.

Other objects, features and advantages of the present invention willbecome apparent from the following figures, detailed description, andexamples. It should be understood, however, that the figures, detaileddescription, and examples, while indicating specific embodiments of theinvention, are given by way of illustration only and are not meant to belimiting. Additionally, it is contemplated that changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of various products that can be produced fromsyngas.

FIG. 2 is a schematic of a plug-flow reactor system of the presentinvention.

FIG. 3 is a schematic of fluidized bed reactor system of the presentinvention.

FIG. 4 is a schematic of a membrane separation system of the presentinvention.

FIG. 5 is a schematic of a cryogenic distillation system of the presentinvention.

FIG. 6 are plots of the equilibrium composition of different gaseousproducts of the present invention with a feed composition of 1 kmol CO₂(g) and 1 kmol S(g).

FIG. 7 are plots of the equilibrium composition of different gaseousproducts of the present invention with a feed composition of 2 kmol CO₂(g) and 1 kmol S(g).

FIG. 8 are plots of the equilibrium composition of different gaseousproducts of the present invention with a feed composition of 4 kmol CO₂(g) and 1 kmol S(g).

FIG. 9 are plots of the equilibrium composition of different gaseousproducts of the present invention with a feed composition of 6 kmol CO₂(g) and 1 kmol S(g).

FIG. 10 are bar graphs of equilibrium composition of product gases ofthe present invention at 918° C. and 1 bar with four different feed gascompositions.

FIG. 11 are bar graphs of equilibrium composition of product gases ofthe present invention at 1220° C. and 1 bar with four different feed gascompositions.

FIG. 12 are bar graphs of equilibrium composition of product gases ofthe present invention at 1500° C. and 1 bar with four different feed gascompositions.

FIG. 13 are bar graphs of the ratio of CO/SO₂ in equilibrium mixture ofthe present invention at three different temperatures and four differentfeed compositions.

FIG. 14 are bar graphs of the ratio of CO/COS in the equilibriumreaction mixture of the present invention at three differenttemperatures and four different feed compositions.

FIG. 15 are bar graphs of the ratio of CO₂/(CO+SO₂) in equilibriummixture of the present invention at three different temperatures andfour different feed compositions.

FIG. 16 shows graphs of the effect of COS recycling the presence of aMo₂S catalyst at two different temperatures in a system of the presentinvention.

FIG. 17 shows graphs of the effect of COS recycling in the presence of aMo₂S catalyst at two different temperatures in the present invention ina system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a solution to the current problemsassociated with converting carbon dioxide to carbon monoxide. Thesolution resides in reacting gaseous sulfur with carbon dioxide toproduce carbon monoxide and sulfur dioxide, which is represented byequation (6) shown above. The reaction can be tuned via the reactiontemperature and amounts of reactants used to obtain a particular productstream profile. For instance, other reaction products that can beproduced during the reaction include COS(g), S(g), and CS(g). Each ofthe reaction products can be further processed into desired chemicals.By way of example, the produced carbon monoxide can be converted tosyngas by converting part of the carbon monoxide to into hydrogen gas bythe water gas shift reaction (see equation (5)). Syngas can be used in avariety of processes to produce desired chemicals, examples of which areprovided in FIG. 1. The produced SO₂ can be converted into SO₃ and thensulfuric acid and ultimately ammonium sulfate fertilizers. Similarly,COS(g) and S(g) can be converted into valuable commercial products orused as reactants to produce more carbon monoxide. These and othernon-limiting aspects of the present invention are discussed in furtherdetail in the following sections.

A. Reaction Feed

The reactant mixture or feed in the context of the present invention caninclude a gaseous mixture that includes, but is not limited to, sulfurgas (S(g)), and carbon dioxide gas (CO₂(g)). Alternatively, the S(g) andCO₂(g) feeds can be introduced separately and mixed in a reactor. Sulfurgas (S(g)) in the context of the present invention can be referred to aselemental sulfur and can include, but is not limited to, all allotropesof sulfur (i.e., S_(n) where n=1 to co). Non-limiting examples of sulfurallotropes include S, S₂, S₄, S₆, and S₈, with the most common allotropebeing S₈. Sulfur gas can be obtained by heating solid or liquid sulfurto their boiling points of about 445° C. Solid sulfur can contain either(a) sulfur rings, which may have 6, 8, 10 or 12 sulfur atoms, with themost common form being S₈, or (b) chains of sulfur atoms, referred to ascatenasulfur having the formula S. Liquid sulfur is typically made up ofS₈ molecules and other cyclic molecules containing a range of six totwenty atoms. Solid sulfur is generally produced by extraction from theearth using the Frasch process, or the Claus process. The Frasch processextracts sulfur from underground deposits. The Claus process producessulfur through the oxidation of hydrogen sulfide (H₂S). Hydrogen sulfidecan be obtained from waste or recycle stream (for example, from a planton the same site, or as a product from hydrodesulfurization of petroleumproducts) or recovery the hydrogen sulfide from a gas stream (forexample, separation for a gas stream produced during production ofpetroleum oil, natural gas, or both). A benefit of using sulfur as astarting material is that it is abundant and relatively inexpensive toobtain as compared to hydrogen gas.

Carbon dioxide used in the present invention can be obtained fromvarious sources. In one non-limiting instance, the carbon dioxide can beobtained from a waste or recycle gas stream (e.g. from a plant on thesame site, like for example from ammonia synthesis) or after recoveringthe carbon dioxide from a gas stream. A benefit of recycling such carbondioxide as a starting material in the process of the invention is thatit can reduce the amount of carbon dioxide emitted to the atmosphere(e.g., from a chemical production site).

The reactant mixture may further contain other gases, provided thatthese do not negatively affect the reaction. Examples of such othergases include nitrogen or argon. In some aspects of the invention, thereactant gas stream is substantially devoid of other reactant gas suchas hydrocarbon gases, oxygen gas, hydrogen gas, water or any combinationthereof. Hydrocarbon gases include, but are not limited to, C₁ to C₅hydrocarbon gases, such as methane, ethylene, ethane, propane,propylene, butane, butylene, isobutene, pentane and pentene. In aparticular aspect of the invention the gaseous feed contains 0.1 wt. %or less, or 0.0001 wt. % to 0.1 wt. % of combined other reactant gas. Inthe reactant mixture, a molar ratio of CO₂(g) to S(g) can range from 1:1to 6:1 and any range therein. Ratios lower than 1:1 and higher than 6:1are also contemplated in the context of the present invention.Ultimately, the ratio can be varied to produce a desired reactionproduct profile.

B. Reaction Products

The products made from the reduction of carbon dioxide with sulfur inthe gas phase can be varied by adjusting the molar ratio of CO₂(g) toS(g), the reaction conditions, or both. The major products produced fromthe reaction of carbon dioxide and sulfur is carbon monoxide and sulfurdioxide as shown in reaction equation (6). The other products that canbe produced by the reaction include CS₂ and COS as shown in equation(7), with 10% or less of the reaction product being CS₂ at any ratio ofCO₂ to S. In some aspects of the invention, the distribution of productsin the product stream (for example, COS(g), SO₂, CS₂, CO₂, CO and SO₂)can be controlled by adjusting the ratio of carbon dioxide to sulfurfrom 1:1 to 2:1 and up to 6:1 and the temperature of the reaction.

CO₂(g)+S(g)→COS(g)+SO₂(g)+CS₂(g)+CO(g)  (7)

1. COS Formation

Without wishing to be bound by theory, it is believed that, as shown inequation (8), carbon dioxide initially reacts with sulfur to formcarbonyl sulfide and oxygen. In some aspect of the invention, the amountof COS(g) produced can be adjusted by varying the temperature of thereaction. At a temperature 400 and 700° C., the product stream containsCOS and SO₂ with a minimal amount of CO. At these temperatures, theratio of COS:SO₂ can be 2:1 or 1:1. In some aspects of the invention,the COS can be separated from the SO₂ and CO₂ as described throughoutthis Specification and sold or further processed into other chemicalproducts.

2CO₂(g)+2S(g)→2COS(g)+O₂(g)  (8)

2. CO and SO₂ Formation

Without wishing to be bound by theory, it is believed that the carbonylsulfide and oxygen in equation (8) react with carbon dioxide and sulfurto form SO₂ and CO as shown in equations (9) and (10). In some aspectsof the invention, CO and SO₂ are produced at temperatures between 700and 3000° C., 900 to 2000° C., or 1500 to 1700° C., with a preferredtemperature of between 1000 and 1600° C. and CO₂ to S ratios of 1:1 to2:1, and up to 6:1. In other instances, however, lower temperatures arealso contemplated (e.g., 250° C. or more or certain temperature andpressure conditions can be used to ensure sulfur is in the gaseousphase—e.g., conditions at which substantial vapor pressure of S exists,e.g., vapor pressure of S is 5×10⁻⁴ atm at 119° C. and 1 atm at 444.6°C.). The ratio of CO(g) to SO2(g) in the product mixture can range from0.1:1, 1:2, 1:1, 2:1. The temperature of the reaction and/or CO₂/S ratiocan be adjusted to produce a desired CO/SO₂ ratio. For example, if ahigh CO/SO₂ is desired, a temperature of 1200° C. can be used instead of1500° C. On the other hand, if a high CO/COS ratio is desired, a CO₂/Sratio of 6:1 and temperature of 1500° C. or 1200° C. can be used. The ofequilibrium ratios of CO(g) to SO2(g) at 918° C., 1120° C. and 1500° C.and different temperatures are summarized in Table 1.

S((g))+O₂((g))→SO₂((g))  (9)

COS((g))+2CO₂((g))→SO₂((g))+3CO((g))  (10)

TABLE 1 CO₂:S CO:SO₂ ratio at CO:SO₂ ratio at CO:SO₂ ratio at ratio 918°C. 1120° C. 1500° C. 6:1  0.8:1 1.78:1   2:1 4:1 0.55:1 1.55:1 1.9:1 2:1 0.3:1  1.1:1 1.6:1 1:1  0.1:1  0.9:1 0.5:1

A ratio of CO/COS at about 900° C. is about 120:1 with a starting CO₂ toS ratio of 6:1. Equilibrium ratio of CO₂ to the combined CO and SO₂ issummarized in Table 2.

TABLE 2 CO₂/(CO + SO₂) CO₂/(CO + SO₂) CO₂/(CO + SO₂) CO₂:S ratio atratio at ratio ratio 918° C. 1120° C. at 1500° C. 6:1 6.2:1   2:1 1.2:14:1 4.9:1 1.5:1 0.8:1 2:1 2.8:1 0.8:1 0.3:1 1:1   1:1 0.1:1 0.5:1

Without wishing to be bound by theory, it is believed that attemperatures above 1500° C., additional CO(g) is formed through thedecomposition of any remaining COS to CO(g) and S(g) as shown inequation (11). In embodiments when the CO₂ to S ratio is greater than2:1, the COS(g) decomposition can be suppressed.

COS((g))→CO((g))+S((g))  (11)

3. CS₂ Formation

In certain aspects of the invention when the ratio of CO₂ to S is 1:1 or2:1, and the temperature of the reaction is from about 445 to about 700°C., the amount of CS₂ formed as shown in equation (12). The amount ofcarbon disulfide produced can be about 10% or less on a molar basis. Theoxygen produced can react with sulfur to form sulfur dioxide.

CO₂((g))+2S((g))→CS₂((g))+O₂((g))  (12)

In some aspects of the invention, to inhibit or reduce the amount ofcarbon disulfide formation, the amount of CO₂ can be increased in thereaction mixture. Without wishing to be bound by theory, it is believedthat the increased CO₂ reacts with the CS₂ to give CO and SO₂ at higherconcentrations of CO₂. In some aspects of the invention, at a CO₂:Sratio of 4:1, no, or undetectable amounts of, CS₂ is formed attemperatures between 400 to 3000° C. It is believed that at temperaturesgreater than 1000° C., any carbon disulfide that is generated decomposesto carbon monosulfide CS(g) and S(g). The generated sulfur can reactwith excess carbon dioxide to continue production of COS, CO and SO₂.Without wishing to be bound by theory, it is believed that the carbonmonosulfide can polymerize at reaction temperatures above 1000° C.

C. Process

The reaction of carbon dioxide and sulfur can be performed at conditionsto produce a product stream that includes carbonyl sulfide, carbonmonoxide and sulfur dioxide. Non-limiting examples of process for thereduction of carbon dioxide to carbon monoxide in the presence of sulfurare illustrated with reference to the Figures.

1. Reactor Systems

FIGS. 2 and 3 are schematics of reactor systems 100 and 200 of thepresent invention. The reactors used for the present invention can befixed-bed reactors, stacked bed reactors, fluidized bed reactors, slurryor ebullating bed reactors, spray reactors, or plug flow reactor. Thereactors can be manufactured from material resistant to corrosion fromsulfur and/or carbon dioxide. A non-limiting example of such material isstainless steel. In FIG. 2 a plug flow type reactor 102 is depicted andin FIG. 3, a fluidized bed reactor 202 is depicted. Referring to FIG. 2,sulfur is provided to storage vessel 104 as molten sulfur. In someaspects, solid sulfur is heated in storage vessel 104 to about 250° C.to liquefy the molten sulfur. Storage vessel may be to 250 to 300° C. tomaintain the sulfur in a liquid phase. Molten sulfur can exit storagevessel 104 through outlet 106, and be pumped through conduit 108 toreaction vessel inlet 110 at the top of the reactor 102 using pump 112.The outlet 106, the conduit 108, and the reaction vessel inlet 110 canbe heated to 250 to 300° C. to inhibit solidification of the moltensulfur in the outlet, conduit, or inlet. Flow of the molten sulfur intoreaction vessel inlet 110 can be altered using flow switch 114. As shownin FIG. 2, the flow switch 114 is in a disabled or unconnected positionwhich inhibits the molten sulfur from flowing into reaction vessel inlet110. When flow switch 114 is engaged or connected, the molten sulfurflows from conduit 108 to reaction vessel inlet 110. Reaction gases canbe stored in a gas storage unit 116. Reaction gas (for example, carbondioxide or a mixture of carbon dioxide or carbonyl sulfide can exit thegas storage unit 116 through a gas outlet 118, flow through a gasconduit 120, and enter the reaction vessel inlet 110. The gas conduit120 may include a flow switch 122. As shown in FIG. 2, the flow switch122 is in a disabled or unconnected position which inhibits the reactiongases from flowing into reaction vessel inlet 110. When the flow switch122 is engaged or connected, the reaction gases flow from the conduit108 to the reaction vessel inlet 110. The reaction vessel inlet 110 cancouple to a nozzle 124 positioned inside the reactor 104. The nozzle 124can be any known nozzle suitable for providing an aerosol or mist to theinside of the reactor 104. The reaction vessel inlet 110 and the spraynozzle 124 can be heated to 250 to 400° C. As the molten sulfur andreaction gas enter the spray nozzle 114, the compounds are mixed andsprayed as an aerosol into a reaction zone of the reactor 104. Reactor104 can be heated to above the boiling point of sulfur, for exampleabove 415° C. As the aerosol mixture of sulfur and reaction gas entersthe reactor 104, the sulfur vaporizes or transforms into a gas phase.The gaseous sulfur and reaction gases react in the reaction zone ofreactor 104 to form the reaction products described throughout theSpecification. For example, gaseous sulfur reacts with carbon dioxide inthe reaction zone to form a gaseous mixture. The gaseous mixture caninclude CO(g), SO₂(g), COS(g), or any combination thereof. In someinstances, gaseous sulfur is also in the produced gaseous mixture. Asshown, the reactor 102 does not include a catalyst. In some aspects ofthe invention, the reactor 102 may include one or more catalyststhroughout the Specification positioned in the reaction zone described.The gaseous mixture can flow through the reactor 102 and contacts thecatalyst in the reaction zone. Such contact can produce the gaseousmixture.

The gaseous mixture can exit the reactor 104 through reactor outlet 126through gas conduit 128 to condenser 130. Conduit 128 can include one ormore valve 132. Valves 132 may be capable to route a portion of thegaseous mixture to analyzer 134. For example, valves 132 may bethree-way valves. Analyzer may be any suitable instrument capable ofanalyzing a gaseous mixture. A non-limiting example of an analyzer is agas chromatograph in combination with a mass spectrometer (GC/MS). Thecondenser 130 may cool the gaseous mixture to a temperature suitable tocondense sulfur dioxide, gaseous sulfur, if present, or both from thegaseous mixture. Condenser 130 may be part of a recovery unit thatseparates the components of the gaseous mixture. Such a recovery unit isdescribed in more detail in the following sections.

Referring to FIG. 3, a schematic of fluidized bed reactor system 200 isdepicted. The system 200 includes reactor 202, catalyst treatment unit204, and sublimation unit 206. In the reactor system 200, solid sulfurmay be provided to a sublimation unit 206 through sublimation inlet 208.In sublimation unit 206, the sulfur is heated to about 100° C. to allowthe sulfur to sublime into catalyst treatment unit 204 throughsublimator 210. In the catalyst treatment unit 204, the sublimatedsulfur contacts the catalyst and adsorbs onto the catalysts. Contact ofthe sulfur with the metals in the catalysts activates the metals in thecatalyst. The activated catalyst exits the catalyst treatment unit 204through catalyst treatment unit outlet 212 of the reactor 202 throughreactor catalyst inlet 214. Reactant gas (carbon dioxide) enters reactor202 through reactor gas inlet 216 under pressure. The pressure of thereaction gas is sufficient to move the catalysts in an upwardlydirection in the reactor 202 and mix the catalyst with the reaction gas.As the mixture of reaction gas and catalysts enters a reaction zone 218,the sulfur reacts with the reactant gas to form a gaseous productmixture that includes CO₂, SO₂, COS, or combinations thereof. Thereaction zone 218 can be heated to 500 to 1500° C., which can acceleratethe reaction between the absorb sulfur and the reaction case. Thegaseous mixture can exit the reactor 202 through reactor gas outlet 220and be transported to one or more recovery systems. Spent catalyst mayexit the reactor 202 through reactor catalyst outlet 222 and entercatalyst treatment unit 204 through catalyst treatment inlet 224. Incatalyst treatment unit 204, the catalyst is contacted with freshsublimated sulfur and the cycle is repeated.

2. Product Recovery Systems

In some aspects of the process, the components of the gaseous productmixture can be separated into sulfur, sulfur dioxide, carbonyl sulfide,carbon monoxide or combinations thereof using known separationtechnology methods. In some embodiments, thermal-based separationsystems (e.g. condensation, distillation) can be used to remove eachcomponent and produce a pure stream of CO. Other forms of separation,such as chemi- and physi-sorption systems can also be used to removeparticular components. For example, carbon dioxide (CO₂) can be removedusing amine based chemi-sorption. Carbonyl sulfide (COS) can be removedusing an aqueous treatment system. In some embodiments, the products canbe separated using a membrane system or a cryogenic distillation system.FIGS. 4 and 5 are schematics of non-limiting examples of recovery orseparation systems. FIG. 4 is a schematic of a membrane separationsystem. FIG. 5 is a schematic of a cryogenic distillation system. Asshown, in FIGS. 4 and 5 the flow of gas is upwardly through the reactor,however, it should be understood that a linear flow reactor or a reactorwith downwardly flow can be used.

3. Membrane Separation System

Referring to FIG. 4, membrane separation system 300 includes a reactor302, heat exchangers 304 and 306, condenser 308, membrane separationunit 310, and scrubber 312. Gaseous reactant stream 314 that includesS(g) and CO₂(g) enters reactor 302 through reactor inlet 316. Flow ofthe gaseous reactant stream can be regulated using valve 318. Valve 318can be a mixing valve or 3-way valve that allows other streams to mixwith the gaseous reactant stream 314 as they enter the reactor 302. Insome embodiments, a gaseous sulfur stream and a gaseous carbon dioxidestream enter valve 318 or reactor 302 through separate inlets. Inreactor 302, the gaseous reactant stream 314 is heated at temperaturesand pressures described throughout this Specification to produce thegaseous product stream 320. In some embodiments, the gaseous reactantstream 314 is contacted in a reaction zone with a catalyst describedthroughout this Specification under sufficient conditions to produce thegaseous product stream 320. The gaseous product stream 320 can includegaseous carbon monoxide, gaseous carbonyl sulfide, and gaseous sulfurdioxide. In some embodiments, the gaseous product stream includesgaseous carbon disulfide and gaseous sulfur. The gaseous product stream320 can pass through heat exchangers 304 and 306 in a sequential mannerand undergo multiple heat exchanges to reduce the temperature of productstream 320. The cooled gaseous product stream 320 can enter thecondenser 308, which is at a temperature sufficient to separate liquidSO₂ from the gaseous product stream 320 and form liquid sulfur dioxidestream 322 and gaseous product stream 324. In some embodiments, thetemperature of the condenser ranges from −150 to −55° C. The liquidsulfur dioxide stream 322 exits condenser 308 and passes through heatexchanger 306 to produce a gaseous sulfur dioxide stream 326. In heatexchanger 306, heat transfer between hot gaseous product stream 320 andliquid sulfur dioxide stream 322 can be sufficient to gasify all, orsubstantially all, of the sulfur dioxide in the sulfur dioxide stream326. The gaseous sulfur dioxide stream 326 can be transported to storageunits, transported to other processing units to be converted into othercommercial products, and/or sold.

The gaseous product stream 324 can exit condenser 308, pass through theheat exchanger 304, the compressor 328, and then enter membrane unit310. As the gaseous product stream 324 passes through heat exchanger304, the gaseous product stream 324 is heated by exchange of heat withthe hot gaseous product stream 320. Compression of the heated gasproduct stream 324 can further heat the gaseous product stream 324 to adesired temperature for separation in membrane separation unit 310. Insome embodiments, the compressor 328 is not necessary. The heatedgaseous product stream 324 enters the membrane separation unit 310through a feed inlet 330. In the membrane separation unit 310, carbonylsulfide can be separated from the gaseous product stream 324 to form acarbonyl sulfide stream 332 and a gaseous carbon monoxide stream 334. Aportion of the gaseous carbonyl sulfide stream 332 can be transported toother units or to storage units, or sold through conduit 336. A portionof the gaseous carbonyl stream 332 can be provided to the valve 318,mixed with the gaseous reactant stream 314 and feed to reactor 316. Insome embodiments, a gaseous sulfur stream, a gaseous carbon dioxidestream and a gaseous carbonyl sulfide stream, or combinations thereofare provided directly as single streams or mixtures of streams to thereactor 302. The gaseous carbon monoxide stream 334 can enter thescrubber 312. In the scrubber 312, residual amounts of carbonyl sulfideand/or sulfur dioxide can be removed from the gaseous carbon monoxidestream 334 to produce purified a carbon monoxide stream 338. Thescrubber 312 can be any known scrubber system capable of separating COSand SO₂ from CO. For example, the scrubber 312 may be an aqueoustreatment system. Waste product stream containing carbonyl sulfide,sulfur dioxide, and water can exit the scrubber system 312 through thewaste outlet 336 and disposed of using known disposal methods. Thepurified carbon monoxide stream 338 can exit scrubber 312 throughscrubber outlet 340 and be transported to other units for furtherprocessing into commercial products, stored, or sold.

4. Cryogenic Separation System

Referring to FIG. 5, cryogenic separation system 400 includes thereactor 302, heat exchangers 304, 306, and 402, the condenser 308, andthe cryogenic separation unit 404. The gaseous reactant stream 314 thatincludes S(g) and CO₂(g) enters the reactor 302 through the reactorinlet 316. Flow of the gaseous reactant stream can be regulated usingthe valve 318 as described above. In the reactor 302, the gaseousreactant stream 314 is heated at temperatures and pressures describedthroughout this Specification to produce a product stream 320. Thegaseous product stream 320 can pass through the heat exchangers 304 and306 to undergo multiple heat exchanges to reduce the temperature of theproduct stream 320. The gaseous product stream 320 can enter thecondenser 308, which is at a temperature sufficient to separate liquidSO₂ from the gaseous product stream 320 and form the liquid sulfurdioxide stream 322 and the gaseous product stream 324. In someembodiments, the temperature of the condenser ranges from −150 to −55°C. The liquid sulfur dioxide stream 322 exits the condenser 308 and canundergo heat exchange in the heat exchanger 306 to produce the gaseoussulfur dioxide stream 326. In heat exchanger 306, the hot gaseousproduct stream 320 can be used as the working fluid to provide heat tothe liquid sulfur dioxide stream 322 to sufficiently to gasify all, orsubstantially gasify all, of the liquid sulfur dioxide in the sulfurdioxide stream 326 to gaseous sulfur dioxide. The gaseous sulfur dioxidestream 326 can be transported to storage units, transported to otherprocessing units to be converted into other commercial products, and/orsold.

The gaseous product stream 324 can exit condenser 308 and pass throughthe heat exchanger 402. Heat exchange in the heat exchanger 402 can coolthe gaseous product stream 324. For example, the temperature of theworking fluid in the heat exchanger 308 can be about −50° C. The gaseousproduct stream 324 can enter cryogenic separation unit 404 throughcryogenic separation inlet 406. In some embodiments, heat exchanger 402is not used, and gaseous product stream 324 enter cryogenic separationinlet 410. In cryogenic separation unit 404, carbon monoxide isseparated from gaseous product stream 324 to form a carbon monoxidestream 408. The cryogenic separation unit 404 may have 2 to 100, 20 to50, or 30 to 40 distillation plates and be operated at temperatures andpressures sufficient to separate carbon monoxide from gaseous productstream 324. For example, cryogenic distillation can be operated as atemperature of −140 to −55° C. The purified carbon monoxide stream 408can exit the cryogenic separation unit 404 through a gas outlet 410,pass through heat exchanger 304 and be transported to storage units,other process facilities or sold as a commercial product. Carbonmonoxide stream 408 can have 90 to 100%, or preferably 100% by volumecarbon monoxide. While passing through heat exchanger 304, the coldcarbon monoxide stream 408 may cool the hot gaseous product stream 320exiting reactor 302 and, thus improve the heat efficiency of the system.In some embodiments, the carbon monoxide stream 408 does not passthrough heat exchanger 304. In cryogenic separation unit 404, theconditions are sufficient to liquefy or partially liquefy carbonylsulfide (i.e., at temperatures below the boiling point of carbonylsulfide (about −50° C.) and form a liquid carbonyl sulfide stream 412.The liquid carbonyl sulfide stream 412 can exit the cryogenic separationunit 404 through liquid outlet 414 and pass through heat exchanger 402.In the heat exchanger 402, the liquid carbonyl sulfide stream 412 isgasified to form gaseous carbonyl sulfide stream 416. The heat in heatexchanger 402 can be provided from the gaseous product stream 324, thusmaximizing the heat efficiency of the cryogenic distillation system 400.The gaseous carbonyl stream 416 can enter valve 318 and be mixed with agaseous reactant stream 314 to continue the process cycle. In someembodiments, the gaseous carbonyl stream 416 directly enters the reactor302.

With respect FIGS. 2-5, not all conduits and vessel inlets and outletsare described as it should be understood that the units described in thefigures have inlets, outlets and conduits that in fluid communication.It should also be understood that the arrangement of the components inthe systems can be combined and/or used in a different order.

D. Catalysts and Reaction Conditions

Catalytic material used in the context of this invention may be the samecatalysts, different catalysts, or a mixture of catalysts. The catalystsmay be supported or unsupported catalysts. The support may be active orinactive. The catalyst support can include refractory oxides, aluminaoxides, aluminosilicates, silicon dioxide, metal carbides, metalnitrides, sulfides, or any combination thereof. Non-limiting examples ofsuch compounds includes MgO, Al₂O₃, SiO₂, Mo₂C, TiC, CrC, WC, OsC VC,Mo₂N, TiN, VN, WN, CrN, Mo₂S, ZnS, and any combination thereof. All ofthe support materials can be purchased or be made by processes known tothose of ordinary skill in the art (e.g.,precipitation/co-precipitation, sol-gel, templates/surface derivatizedmetal oxides synthesis, solid-state synthesis, of mixed metal oxides,microemulsion technique, solvothermal, sonochemical, combustionsynthesis, etc.). One or more of the catalysts can include one or moremetals or metal compounds thereof. The metals that can be used in thecontext of the present invention to create bulk metal oxides, bulk metalsulfides, or supported catalysts include a metal from Group IIA orcompound thereof, a metal from Group IB or compound thereof, a metalfrom Group IIIB or compound thereof, a metal from Group IVB or compoundthereof, a metal from Group VIB or compound thereof, a metal from GroupVIII or compound thereof, at least one lanthanide or compound thereof,or any combination thereof. The metals or metal compounds can bepurchased from any chemical supplier such as Sigma-Aldrich® (USA),Alfa-Aeaser (USA), Strem Chemicals (USA), etc. Group IIA metals(alkaline-earth metals) and Group IIA metal compounds include, but arenot limited to, Mg, MgO, Ca, CaO, Ba, BaO, or any combinations thereof.Group IB metals and Group IB metal compounds include, but are notlimited to, Cu and CuO. Group IIB metals include zinc or zinc sulfide.Group IIIB metals and Group IIIB metal compounds include, but are notlimited to, Sc, Sc₂O₃, the lanthanides or lanthanide compounds, or anycombination thereof. Lanthanides that can be used in the context of thepresent invention to create lanthanide oxides include La, Ce, Dy, Tm,Yb, Lu, or combinations of such lanthanides. Non-limiting examples oflanthanide oxides include CeO₂, Dy₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, or La₂O₃, orany combination thereof. Lanthanide oxides can be produced by methodsknown in the art such as by high temperature (e.g., >500° C.)decomposition of lanthanide salts or by precipitation of salts intorespective hydroxides followed by calcination to the oxide form. GroupIVB metals and Group IV metal compounds include, but are not limited to,Zr and ZrO₂. Group VIB metals and Group VI metal compounds include, butare not limited to, Cr, Cr₂O₃, Mo, MoO, Mo₂O₃, or any combinationthereof. Group VIII metals and metal compounds include, but are notlimited to, Ru, RuO₂, Os, OsO₂, Co, Co₂O₃, Rh, Rh₂O₃, Ir, Ir₂O₃, Ni,Ni₂O₃, Pd, Pd₂O₃, Pt, Pt₂O₃, or combinations thereof. The catalyticmaterial can be subjected to conditions that results in sulfurization ofthe metal in the catalytic material. Non-limiting examples of metal thatcan be sulfided prior to use are Co, Mo, Ni and W. The catalyst materialcan, in some instances include a promoter compound. A non-limitingexample of promoter compound is phosphorus. A non-limiting example of acatalyst that includes a promoter compound is catalyst material thatincludes Mo—Ni—P. In some instances, the metal oxides described hereincan be of spinel (general formula: M₃O₄), olivine (general formula:M₂SiO₄) or perovskite (general formula: M¹M²O₃) classification.

The catalyst used in the present invention is sinter and coke resistantat elevated temperatures, (e.g., 445° C. to 3000° C., 900 to 2000° C.,or 1000 to 1600° C.). Further, the produced catalysts can be usedeffectively in reaction of sulfur with carbon dioxide at a pressure of 1to 25 bar, and/or at a gas hourly space velocity (GHSV) range from 1000to 100,000 h⁻¹.

E. Further Processing of Products

1. CO Processing

The carbon monoxide produced using the method of the invention can bepartially converted into H₂ through water gas shift reaction for theproduction of syngas of desired H₂/CO ratio as shown in equation (13).The produced CO₂ can be used in the current process to produce morecarbon monoxide. This provides an efficient, economic, and novel methodto convert a greenhouse gas (CO₂) into value added and useful products.

CO+H₂O→H₂+CO₂  (13)

2. SO₂ Processing

The sulfur dioxide produced using the method of the invention can beconverted to SO₃, which can be further processed into sulfuric acid andammonium sulfate as shown in the equations (14) through (17).

SO₂+½O₂→SO₃  (14)

SO₃+H₂SO₄→H₂S₂O₇  (15)

H₂S₂O₇+H₂O→2H₂SO₄  (16)

2NH₃+H₂SO₄→(NH₄)₂SO₄  (17)

3. COS Processing

The carbonyl sulfide produced using the method of the invention can beused in the production of thiocarbamates. Thiocarbamates can be used incommercial herbicide formulations. The method of the invention providesan advantage over commercially prepared COS, which is synthesized bytreatment of potassium thiocyanide and sulfuric acid as shown inequation (18).

KSCN+2H₂SO₄+H₂O→KHSO₄+NH₄HSO₄+COS  (18)

The conventional treatment produces potassium bisulfate and ammoniumbisulfate which needs to be separated, which is a difficult and timeconsuming process. The method of the invention provides an efficient andeconomic method solution to the production of COS.

EXAMPLES

The present invention will be described in greater detail by way ofspecific examples. The following examples are offered for illustrativepurposes only, and are not intended to limit the invention in anymanner. Those of skill in the art will readily recognize a variety ofnoncritical parameters which can be changed or modified to yieldessentially the same results.

Example 1 Equilibrium Calculations of Reactions

Multiphase equilibrium composition calculation was done by using HSCChemistry 7.1 software (Outotec Oyi, Espoo, Finland). The parametersused in the calculations were ratios of gaseous carbon dioxide togaseous sulfur ranging from 6:1 to 1:1 at temperatures between 0-3000°C. FIGS. 6-9 are graphs of the calculated equilibrium compositionobtained by treating different ratios of CO₂ with gaseous S attemperatures from 0 and 3000° C. FIGS. 10-12 are graphs of the amount ofdifferent gaseous species at equilibrium conditions between fourdifferent feed ratios of CO₂/S were compared at three differenttemperatures. FIGS. 13-15 are bar graphs of various product ratios inthe equilibrium reaction mixture at three different temperatures andfour different feed compositions. FIG. 5 are plots of the equilibriumcomposition of different gaseous species with a feed composition of 1kmol CO₂ (g) and 1 kmol S(g). FIG. 7 are plots of the equilibriumcomposition of different gaseous species with a feed composition of 2kmol CO₂ (g) and 1 kmol S(g). FIG. 8 are plots of the equilibriumcomposition of different gaseous species with a feed composition of 4kmol CO₂ (g) and 1 kmol S(g). FIG. 9 are plots of the equilibriumcomposition of different gaseous species with a feed composition of 6kmol CO₂ (g) and 1 kmol S(g). The calculated results demonstrate thatgaseous ‘S’ reacts with CO₂ to form equilibrium mixture of SO₂, CO, CS₂and COS in different amounts at different temperature. Referring toFIGS. 6 and 7, it was found that when the CO₂/S ratio was 1 and 2, smallamount of CS₂ forms. Referring to FIGS. 8 and 9, it was found that whenthe CO₂/S ratio was 4 to 6, CS₂ did not form at any temperature. It wasalso found that SO₂ formed between 0-3000° C. in considerable quantitybetween 0-3000° C. when the CO₂/S ratio is 1, but as the ratio increasedfrom 2-6, the amount of SO₂ below 1000° C. became nearly ⅓ of that of athigh temperature of above 1500° C.

The amount of different gaseous species at equilibrium conditionsbetween four different feed ratios of CO₂/S were compared and plotted inFIGS. 10-15. FIG. 10 are bar graphs of equilibrium composition ofproduct gases at 918° C. and 1 bar with four different feed gascompositions. FIG. 11 are bar graphs of equilibrium composition ofproduct gases at 1220° C. and 1 bar with four different feed gascompositions. FIG. 12 are bar graphs of equilibrium composition ofproduct gases at 1500° C. and 1 bar with four different feed gascompositions. FIG. 13 are bar graphs of the ratio of CO/SO₂ inequilibrium mixture at three different temperatures and four differentfeed compositions. FIG. 14 are bar graphs of the ratio of CO/COS in theequilibrium reaction mixture at three different temperatures and fourdifferent feed compositions. FIG. 15 are bar graphs of the ratio ofCO₂/(CO+SO₂) in equilibrium mixture at three different temperatures andfour different feed compositions. From the obtained data, reactiontemperatures and the CO₂/S ratio can be determined to produce thedesired products. Referring to FIG. 13, it was determined that to obtainhigh CO/SO₂ at a CO₂/S ratio of 1:1, reaction temperatures of 1200° C.are preferred. Referring to FIG. 14, it was determined that to obtainhigh CO/COS ratio, it is preferable to have a CO₂/S ratio of 6 andtemperature of 1500° C. or 1200° C. Referring to FIG. 15, theCO₂/(CO+SO₂) ratio can be altered depending upon the final applicationthis can be applied.

The equilibrium calculations and the obtained results demonstrate thatthe method and systems of the present invention reaction of gaseouscarbon dioxide and gaseous sulfur provides carbon monoxide and sulfurdioxide in an efficient manner and converts a greenhouse product intouseful commercial products.

Example 2 Production of CO and SO₂ with Recycle of COS

General Procedure.

Experiments were conducted at 800° C. and 900° C. in a saturator reactorusing MoS₂ and ZnS catalysts to produce CO and SO₂ with recycle COS tolimit the further production of COS. A gaseous mixture of CO₂ (25ml/min) Ar (25 ml/min) was passed through the saturator reactorcontaining molten sulfur held at 180° C. The sulfur saturated gaseousCO₂ and Ar mixture was passed over the MoS₂ or ZnS catalyst (500 mg)held at 800° C. and 900° C. Both ZnS and MoS₂ were procured fromSigma-Aldrich®, USA. Table 3 lists the physical and kinetic parametersof the catalysts and the reactions.

TABLE 3 Parameters ZnS MoS₂ Surface area (BET) 20.7 m²/g 3.6 m²/g Porevolume (BJH) 0.082 cm³/g 0.026 cm³/g Pore diameter(4V/A) (BJH) 14.0 nm30.4 nm Ea, kJ/mol 27.5 30 R_(CO2), mol/g.s (900° C.) 5.63 × 10⁻⁷ 5.43 ×10⁻⁷ Ea = activation energy and Rco₂ = rate of CO₂ decomposition

MoS₂ Catalyst.

In the saturation reactor, the temperature was raised to 900° C. inpresence of CO₂ and S mixture and held at that temperature for about 1.5hrs to produce COS (4000 ppm), CO (7000 ppm) and SO₂ (5000 ppm). Theproduced COS (4000 ppm) was recycled through the saturation reactor.Results from the production of CO and SO₂ from a MoS₂ catalyst withrecycle of COS at two different temperatures is shown in FIG. 16. Thetop line is CO production and the bottom line is COS production. Theproduct stream was further monitored for 45 minutes and it wasdetermined (See, FIG. 16, time frame between 2 and 3 hours) that theaddition of COS in feed steam did not result in an increase in the COSconcentration in the product stream. Thus, recycling COS at 900° C. overthe MoS₂ catalyst inhibited formation of more COS due to reactionbetween CO and S.

To determine the effect of the catalyst on the COS production at lowertemperatures, the addition of 4000 ppm COS to the CO₂ and S feed streamwas stopped, the reactor temperature was reduced to 800° C., and productgas stream was monitored for about 60 min. At this condition, nearly COS(1800 ppm) was produced. The produced COS (1800 ppm) was fed through thereactor and outlet gas stream was monitored for 50 minutes. It wasobserved that the COS concentration in the outlet stream graduallyincreases over time. (See, FIG. 16, time frame between 4 and 5 hours).Thus, recycling COS at 800° C. did not inhibit formation of COS due tothe reaction between CO and S.

ZnS Catalyst.

In the saturation reactor, the temperature was raised to 900° C. inpresence of CO₂ and S mixture and held at that temperature for about 1.1hrs to produce COS (3600 ppm), CO (7200 ppm) and SO₂ (5140 ppm). Theproduced COS (3600 ppm) was recycled through the saturation reactor forResults from the production of CO and SO₂ from a ZnS catalyst withrecycle of COS at two different temperatures is shown in FIG. 17. Thetop line is CO production and the bottom line is COS production. Theproduct stream was further monitored for 20 minutes and it wasdetermined (See, FIG. 16, time frame between 2 and 3 hours) that theaddition of COS in feed steam did not result in an increase in the COSconcentration in the product stream. Thus, recycling COS at 900° C. overthe ZnS catalyst inhibited formation of more COS due to reaction betweenCO and S. After 20 minutes, the COS concentration was doubled to 7200ppm instead of 3600 ppm and outlet gas stream was monitored for 60minutes. At these conditions, the COS concentration increased and COconcentration decreased with time. These results proved that havingequilibrium amount of COS in the feed stream overcomes further formationof COS due to the reaction between CO and S, and having more than anequilibrium amount hinders formation of CO as well.

To determine the effect of the catalyst on the COS production at lowertemperatures, the addition of COS (7500 ppm) to the CO₂ and S feedstream was stopped, the reactor temperature was reduced to 800° C., andproduct gas stream was monitored for about 60 min. At this condition,nearly COS (2600 ppm) was produced. The produced COS (2600 ppm) was fedthrough the reactor and outlet gas stream was monitored for 30 minutes.It was observed that the COS concentration in the outlet streamgradually increases over time. (See, FIG. 17, time frame between 4 and 3hours). Thus, recycling COS at 800° C. did not inhibit formation of COSdue to the reaction between CO and S.

In summary, ZnS and MoS₂ were found to catalytically dissociate CO₂ attemperature above 600° C. in presence of metallic sulphur. COS was amajor byproduct and its production was controlled by separation andrecycling. Recycling the COS over ZnS or MoS₂ at 900° C. overcamefurther formation of COS due to the reaction between CO and S.

1. A method of producing carbon monoxide (CO) and sulfur dioxide (SO₂),the method comprising: (a) obtaining a reaction mixture comprisingcarbon dioxide gas (CO₂(g)) and elemental sulfur gas (S(g)); and (b)subjecting the reaction mixture to conditions sufficient to produce aproduct stream comprising CO(g) and SO₂(g).
 2. The method of claim 1,wherein the product stream further comprises carbonyl sulfide (COS(g)).3. The method of claim 2, wherein the product stream further comprisescarbon disulfide (CS₂(g)).
 4. The method of claim 1, wherein the productstream further comprises CO₂(g) and S(g).
 5. The method of claim 4,wherein the product stream consists essentially of or consists of CO(g),SO₂(g), COS(g), CO₂(g), and S(g) or CO(g), SO₂(g), COS(g), CS₂(g),CO₂(g), and S(g).
 6. The method of claim 1, wherein the reaction mixturecomprises a CO₂(g):S(g) molar ratio of 1:1 to 6:1.
 7. The method ofclaim 6, wherein the reaction mixture comprises a CO₂(g):S(g) molarratio of 4:1.
 8. The method of claim 7, wherein the product stream doesnot include CS₂(g).
 9. The method of claim 6, wherein the reactionmixture comprises a CO₂(g):S(g) molar ratio of 6:1.
 10. The method ofclaim 9, wherein the product stream does not include CS₂(g).
 11. Themethod of any claim 1, wherein the reaction temperature in step (b) isat least 445° C.
 12. The method of claim 11, wherein the reactionpressure is 1 to 25 bar.
 13. The method of claim 12, wherein a gashourly space velocity (GHSV) of 1,000 to 100,000 h⁻¹ is used.
 14. Themethod of claim 1, further comprising contacting the reaction mixture instep (b) with a catalyst.
 15. The method of claim 14, wherein thecatalyst comprises a metal, a metal oxide, a metal sulfide, alanthanide, a lanthanide oxide, or any combination thereof.
 16. Themethod of claim 15, wherein the metal, metal oxide, or metal sulfideincludes a Group IIA, IB, IIB, IIIB, IVB, VIB, or VIII metal.
 17. Themethod of claim 16, wherein the metal sulfide comprises molybdenum orzinc.
 18. The method of claim 15, wherein the lanthanide, or lanthanideoxide includes La, Ce, Dy, Tm, Yb, Lu, CeO₂, Dy₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃,or La₂O₃, or any combination thereof.
 19. The method of claim 14,wherein the catalyst is a bulk metal catalyst.
 20. The method of claim14, wherein the catalyst is a supported catalyst.
 21. The method ofclaim 20, wherein the support comprises a metal sulfide, a metalcarbide, a metal nitride, or a metal phosphate, and any combinationthereof.
 22. The method of claim 1, wherein hydrogen gas, oxygen gas,methane gas, and water are not included in the reaction mixture.
 23. Themethod of claim 1, wherein the product stream comprises CO(g), SO₂(g)and COS(g), and the COS (g) is recycled to step (b) at a reactiontemperature of 900° C. or more.
 24. The method of claim 23, whereinrecycling the COS (g) inhibits formation of additional COS (g).
 25. Asystem for producing carbon monoxide (CO) and sulfur dioxide (SO₂), thesystem comprising: an inlet for a feed comprising a carbon dioxide gas(CO₂(g)) and elemental sulfur gas (S(g)) or a first inlet for a feedcomprising CO₂(g) and a second inlet for a feed comprising S(g); and areactor comprising a reaction zone that is configured to be in fluidcommunication with the inlet or inlets, wherein the reaction zonecomprises CO₂(g) and S(g); and an outlet configured to be in fluidcommunication with the reaction zone to remove a product streamcomprising CO(g) and SO₂(g).
 26. A reaction mixture comprising carbondioxide gas (CO₂(g)) and elemental sulfur gas (S(g)).
 27. A productstream comprising carbon monoxide gas CO(g) and sulfur dioxide gasSO₂(g).